Abstract
Purpose: :
To demonstrate and assess the use of a novel psychophysical technique to measure macular pigment optical density (MPOD) using a CRT display.
Methods: :
The apparent motion photometry technique described by Anstis & Cavanagh (Colour vision: Psychophysics and physiology. London: Academic Press, 1983. pp. 155–166) has been developed to enable rapid measurement of MPOD profiles in naïve subjects. The stimulus is a chromatic blue–red grating and an achromatic luminance grating displayed sequentially with a second pair of similar gratings, each grating being displaced spatially 90 degrees in advance of the previous one. The gratings appear to drift either up or down; the direction of drift being dependant upon the relative perceived luminance of the red and blue component. The subject’s task is simple to understand and perform. Grating sequences are presented on a calibrated CRT display and the relative red and blue luminance is controlled using a 2AFC weighted staircase procedure to determine the point of subjective isoluminance. Isoluminince is determined at a series of retinal eccentricities between 0–7 deg. allowing an MPOD profile to be calculated. Fixation is monitored and stimulus presentation occurs only when fixation is correct.
Results: :
A model was developed from an analysis of the spectral characteristics of CRT phosphors to compensate for their wide band emission spectra. We measured the MPOD profiles in a group of young healthy adults with normal vision and correlated these with personal, environmental and dietary factors. The corrected measures showed good agreement with published values. In subjects that had been measured by other techniques there was also good correlation. For example, a typical subject had a corrected peak MPOD value of 0.41 and profile width (HWFH) of 1.5 deg measured by AMP, compared with 0.35 and 1.5 deg by Heterochromatic Flicker Photometry.
Conclusions: :
We have demonstrated the utility of this technique in measuring MPOD in the general population. The conceptually simple task for the subject improves performance and reduces the variance of MPOD values.
Keywords: macular pigment • retina • carotenoids/carotenoid binding proteins